The production of hydrogen by catalytic steam reforming of a hydrocarbon such as methane is well known in the art. The steam and hydrocarbon reforming reaction is highly endothermic. Therefore, the reactor requires a heat input for the reaction to proceed. In general, processes where the required heat is generated outside the reactor and transferred to the reactor, so-called allothermic processes, are well-known and is usually accomplished by enclosing reactor tubes containing a catalyst within a fired furnace. The use of a furnace results in the generation of much thermal energy which must be recovered for economic reasons. Usually, this leads to the production of excess high-pressure steam which cannot always be properly used. As well, such processes are conventionally carried out at a relatively high temperature, usually between about 800.degree. and 1000.degree. degrees C. which creates metallurgical problems in the construction of the reactor and catalyst pipes.
In U.S. Pat. No. 5,326,550, the contents of which are incorporated herein by reference, Adris el al. teaches a method for producing hydrogen gas in a fluidized bed. In particular, it teaches the use of a circulating thermal fluid in heat pipes or heat exchangers embedded within the fluidized bed of a membrane reactor to maintain the reactor temperature in the required range. While the use of embedded heat pipes or heat exchangers represented an improvement over heating elements external to the reactor, this configuration still presents many drawbacks. The use of an embedded heat exchanger complicates the reactor construction and still requires the generation of heat energy in an external location, which is thermally inefficient and requires a considerable investment of energy and therefore significantly increases the cost of operating the reactor. Furthermore, the fluidized bed may not be completely temperature uniform as there will still be temperature gradients leading away from the heat exchanger tubes.
The Adris patent also teaches the use of a hydrogen-selective permeable membrane to separate hydrogen from the reacting gases. In general, membrane reactors are particularly suited for reactions which are equilibrium limited, as significant enhancement over the equilibrium conversion may be achieved by selectively removing one or more reaction products, such as hydrogen, through the membrane wall.
Processes where the required reaction heat is generated within the reaction system, or autothermal processes, using fixed or packed beds of catalyst are well-known but suffer from some significant disadvantages which tend to prevent their practice on an industrial scale. Isomura et al. in U.S. Pat. No. 5,741,474 alleviated the need for external heating by the addition of oxygen to a fixed bed reactor such that the heat generated by partial oxidation of the hydrocarbon provides heat necessary for reforming. Although this configuration represents an improvement in operational efficiency, the Isomura reactor utilizes a fixed bed configuration in which the rapid combustion reaction is expected to generate large temperature gradients and hot spots which may sinter the catalyst and damage the membrane. In particular, the heat-generating oxidation reaction occurs quickly in a region localized around the oxygen input.
In U.S. Pat. No. 5,714,092 issued to Van Looij et al., an autothermal process is disclosed which utilizes a fixed catalyst beds. A separate oxidation catalyst and reactor is provided in association with a reforming reactor. A separate oxidation catalyst is used to catalyze the oxidation of a hydrocarbon gas feed in order to provide the heat of reaction for the endothermic steam reformation reaction. The oxidation reactor is in-line with the reforming reactor such that the output gases from the oxidation reactor are heated to a sufficient level as they enter the reforming reactor. In general terms, this patent confirms the difficulties associated with previous attempts with internal hydrocarbon oxidation such as the creation of explosive mixtures and the formation of soot. This process attempts to solve these prior art difficulties by separating the oxidation reactor and the reformation reactor and by minimizing the size of the nickel catalyst.
The advantages of a fluidized bed reactor, as opposed to a fixed or packed bed, are well-known and include thermal uniformity, improved heat transfer, catalyst bed uniformity, and substantial elimination of diffusional limitations. However, there have not been any attempts to provide an autothermal fluidized bed reactor because most researchers believed that oxidation reactions required a high initiation temperature such that the hydrocarbon rapidly combusted. It was further believed that maintenance of a stable flame required a higher temperature than that required for the reforming reaction. It was widely reported that the ignition temperature for natural gas or propane was in the range of 800.degree. to about 935.degree. C. Furthermore, it was suspected that the combustion reaction would take place in the freeboard zone, rather than in the fluidized bed, thereby disrupting the reactor process instead of dissipating heat to the reactants in the fluidized bed. Previous research into methane combustion in fluid bed combustors (FBC) indicated that freeboard zone combustion was likely.
Therefore, there is a further need in the art for methods and apparatuses for low-temperature autothermal steam reformation of a hydrocarbon to produce hydrogen which may mitigate the disadvantages of the prior art.